This invention relates to telecommunications systems in which telephone signals are distributed via optical fibers.
U.S. Pat. No. 5,351,234, which has common inventors with the present application, discloses a system in which video signals are distributed, and switched telephone services provided, over a common broadband cable network. The disclosure of U.S. Pat. No. 5,351,234 is incorporated herein by reference.
The above-referenced related patent application, namely Ser. No. 08/726,641, has the same inventors as the ""234 patent. The disclosure of the ""641 application is also incorporated herein by reference.
The ""641 patent application represents an improvement over the cable telephony system of the ""234 patent. Specifically, the ""641 application is concerned with a transmultiplexer, which is a device that provides signal translation between the TDM (time division multiplex) digital signal format employed in a digital telephone switch, and an FDM (frequency division multiplex) signal format employed in the cable transmission system. A salient aspect of the disclosure of the ""641 application calls for the transmultiplexer to provide xe2x80x9cconcentrationxe2x80x9d. That is, the number of subscriber lines served by the transmultiplexer is larger than the number of digital signal channels which interconnect the transmultiplexer with the digital switch.
One of the present inventors has recognized that it may be advantageous to provide telephony and other communication services to subscriber locations by means of dedicated optical communication fibers, with the communication system designed according to a xe2x80x9cstarxe2x80x9d architecture such that each of the optical fibers is dedicated to a single subscriber location, and extends directly between the respective subscriber location and a telephone central office location. Of course, it would be desirable that the switching function at the central office be carried out through conventional digital switching equipment. The question of how to provide a suitable interface between the digital switch and the star-configuration optical fiber network therefore arises.
It is a primary object of the invention to provide a transmultiplexer for translating between the respective signal formats of a digital switch and a dedicated-optical-fiber telecommunications system.
It is a further object of the invention to provide a telecommunications system in which the cost of switching equipment is minimized.
In accordance with the principles of the present invention, the above and other objects are realized by providing a system for transmitting public-switched-telephone-network signals between a digital switch installed at a central office location and a plurality of telephone subscriber locations, wherein the system includes the following components:
(a) a plurality of optical fibers, each interconnecting a respective one of the subscriber locations with the central office location;
(b) a first plurality of optical transceiver devices, each connected to a respective one of the optical fibers at the central office location, wherein each of the transceiver devices includes a facility for transmitting a downstream signal to the respective subscriber location over the respective optical fiber by analog lightwave modulation and for receiving and demodulating an upstream signal transmitted from the respective subscriber premise location over the respective optical fiber by analog light wave modulation; and
(c) a second plurality of optical transceiver devices, each located at a respective one of the subscriber locations and connected to the respective optical fiber, each of the transceiver devices of the second plurality being for transmitting an upstream signal to the central office location over the respective optical fiber by analog lightwave modulation and for receiving and demodulating a downstream signal transmitted from the central office location over the respective optical fiber by analog lightwave modulation; and
(d) a transmultiplexer, which includes:
(1) at least one digital signal port connected to the digital switch for receiving digital telephone signals from the digital switch in a TDM format and for transmitting digital telephone signals in the TDM format to the digital switch, the telephone signals received and transmitted by the at least one digital signal port corresponding to a plurality of voice-grade digital telephone channels;
(2) a plurality of analog signal ports each connected to a respective one of the optical transceiver devices of the first plurality, and each of the analog signal ports being for receiving from the respective optical transceiver device of the first plurality analog telephone signals corresponding to a plurality of voice-grade analog telephone channels in an FDM format and transmitted through the respective optical fiber, each of the analog signal ports also being for transmitting to the respective optical transceiver device of the first plurality analog telephone signals in the FDM format for the plurality of voice-grade analog telephone channels;
(3) circuitry connected between the at least one digital port and the plurality of analog signal ports for translating telephone signals between the TDM and FDM formats; and
(4) a control circuit for selectively coupling any of the voice-grade digital telephone channels to any of the voice-grade analog telephone channels.
One particularly notable feature of the telecommunications system provided according to the invention is that the subscriber side of the transmultiplexer is made up of a number of analog signal ports, each corresponding to and connected to a respective one of the dedicated optical fibers. The same transmission frequency band is preferably used at each of the analog signal ports, so that bandwidth is conserved for other services that may be provided via the optical fibers.
Preferably the transmultiplexer also provides a concentration function, whereby the total capacity of the analog (subscriber-side) signal ports is greater, in terms of voice-grade telephone channels, than the capacity, in terms of digital telephone channels, of the digital signal port or ports. Indeed, concentration by a factor of two or more is contemplated. This saves on the cost of digital switching capacity, including the cost of interface circuits at the digital switch, since the number of subscriber-side ports on the digital switch is less than the number of subscriber telephone lines actually to be served.
As in the above-referenced ""641 application, the concentration function is to be achieved by managing both the transmultiplexer and the digital switch through a separate host computer. The transmultiplexer includes a host interface circuit which receives command messages from the host computer and responds to those messages by assigning subscriber channels to digital switch channels on a per call basis.
According to another aspect of the invention, there is provided a transmultiplexer which includes at least one TDM interface for receiving and transmitting digital telephone signals in a TDM format from and to a digital signal transmission facility, a buffer memory coupled to the at least one TDM interface for temporarily storing the digital telephone signals, translation circuitry coupled to the buffer memory for translating the digital telephone signals between the TDM format and an FDM format, a plurality of FDM ports for receiving and transmitting analog telephone signals in the FDM format, and a plurality of A/D conversion circuits each coupled between a respective one of the FDM ports and the translation circuitry, wherein the A/D conversion circuits convert analog telephone signals in the FDM format into digital telephone signals in the FDM format, and convert digital telephone signals in the FDM format into analog telephone signals in the FDM format. The transmultiplexer further includes a control circuit coupled to the buffer memory, the translation circuit and the A/D conversion circuits for transmitting control signals for the purposes of controlling respective durations of periods during which the buffer memory stores the digital telephone signals in the TDM format, controlling the translation circuit to transmit and receive digital signals in the FDM format to and from selected ones of the A/D conversion circuits, and controlling selected ones of the A/D conversion circuits to receive and transmit digital signals in the FDM format from and to the translation circuitry.